Process for colouring aluminum electrolytically

ABSTRACT

A process for colouring previously anodically-oxidised articles of aluminum or aluminum alloys by the electrolytic treatment of the aluminum oxide layer with alternating current in an acidic, aqueous electrolyte containing metallic salts for colouring and an addition of an amino-alcohol.

United States Patent Zweifel et al.

[4 1 Oct. 14, 1975 PROCESS FOR COLOURING ALUMINUM ELECTROLYTICALLY Inventors: Walter Zweifel, Neuhausen am Rheinfall; Fritz Schneeberger, Schaffhausen, both of Switzerland Swiss Aluminium Ltd., Chippis, Switzerland Filed: Nov. 7, 1974 Appl. No.: 521,664

Assignee:

Foreign Application Priority Data Nov. 9, 1973 Switzerland 15799/73 US. Cl. 204/58; 204/35 N; 204/42 Int. Cl. C25D 11/22 Field of Search 204/35 N, 58, 38 A, 42

References Cited UNITED STATES PATENTS 5/1968 Asada 204/35 N 3,616,297 10/1971 Cooke et a1. 204/35 N 3,616,308 10/1971 Cooke et al. 204/58 3,616,309 10/1971 Asada et a1. 204/58 3,773,631 11/1973 Immel et a1 204/58 3,798,137 3/1974 Kampert 204/35 N 3,849,263 ll/1974 Gedde 204/35 N Primary ExaminerG. L. Kaplan Attorney, Agent, or Firm-Emest F. Marmorek ABSTRACT 7 Claims, No Drawings PROCESS FOR COLOURING ALUMINUM ELECTROLYTICALLY The invention concerns a process for electrolytically colouring anodic oxide coatings on aluminum and aluminum alloys using alternating current.

It is known that oxide layers on Al and Al alloys, manufactured by the DC-sulphuric acid process for example, can be subsequently coloured by an alternating current treatment in acidic solutions of metallic salts. The oxide layer has an effect like a valve on the electrical current with the result that the alternating current is to a large extent rectified to a direct current and in the phase in which the item being treated is cathodic precipitation of metal takes place in the oxide pores as a result of the reduction of the cations.

The pH range of the metal salt solution is of decisive importance for the precipitation of the metal. The resultant free protons have little effect in a strongly acidic range in which the proton concentration is very high. On the other hand in a weakly acidic range in which the proton concentration is about an order of magnitude less, the additional protons have an important effect. Thus in the case of electrolytes of metal salts from which metals are precipitated in weakly acid solutions e.g. in the pH range between 4 and 6, a considerable shift in the pH is to be expected. The buffering with boric acid is not sufficient to prevent a pronounced reduction in the pH value.

The increase in free acid during the colouring process takes place mainly when the other electrodes are inert e.g. of graphite.

Up to now the inequilibrium in the electrode reaction has been treated by the addition of ammonium hydroxt ide or if necessary an alkaline hydroxide to correct the pH value. This has disadvantages however since in certain electrolytes these additions can lead to pitting i.e. undesirable pit or dot shaped marks and to local spalling of the oxide layer.

The object of the invention is to develop a process for colouring previously anodically-oxidised articles of aluminum or aluminum alloys by electrolytic treatment of the aluminum oxide layer with alternating current in an acidic aqueous electrolyte which contains metal salts for colouring, in which process a superior quality of colour can be achieved rapidly and reproducibly. The said process should also eliminate the above mentioned difficulties.

The object is fulfilled by the invention in that an addition of an amino-alcohol is made to the electrolyte. This addition can be made to an electrolyte which is already in service, in order to correct the pH value during the colouring process. The amino-alcohols are added either continuously or periodically until the desired pH value is again reached.

It is an advantage to make an addition of an aminoalcohol at the point of beginning with a new electrolyte. The desired effect can only be achieved however if at least 1 g/l, preferably at least 2 g/l of an amino-alcohol is added. If as a result of this addition the pH value rises too much, then a correction can be made by an addition of sulphuric acid.

The colouring electrolyte can contain all known metal salts, preferably the ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Fe.

The amino-alcohol additions, in particular, mono-, diand tri-ethanol-amine have the advantage that not only can the pH value be raised but that there are then more favourable conditions for the metal ions to be precipitated Thus for example the uniformity of colour in the oxide layer is improved because of the increased conductivity. A further advantage of using additions of amino-alcohols is that with the customary etching times, more intensive and darker colour tones are achieved. For a certain colour tone therefore, a shorter colouring time is needed and/or the voltage of the alternating current can be reduced. Furthermore the electrolyte is less sensitive to the accidental addition of impurities such as alkaline, ammonium and alkaline-earth ions, which means that under normal conditions, also in the case of dark colours, no undesirable pit or dot shaped marks appear.

The coloured aluminum items are preferably but not exclusively used for decorative purposes and used as exteriors in architectural applications.

EXAMPLE 1 Shee samples efan' aluminum alloy of the following composition? 1.4 1.8% Mg, 0.4% Fe, 0.4% Si, 0.1 0.3% Mn, 0.1% Zn, 0.05% Cu with Al having the usual impurities as the rest, were anodised in sulphuric acid and then treated at 20 25C with alternating current in an aqueous colouring electrolyte of the following composition:

g/l CoSO .7l-l O 40 g/l NiSO .7H O

40 g/l H 80 1 g/l CuSO,

A graphite rod, 6 mm in diameter, positioned at a distance of 8 cm from the centre of the sample surface, was used as the other electrode. By connecting the electrodes to a 15 V supply, alternating current, for 120 seconds a medium bronze co1ourtone was obtained on the sample. On the surface of the sample facing the graphite electrode there could be seen an irregularity in the colour, in the form of a dark edge zone.

When the samples were treated under the same conditions but for 15 min with 16 V, alternating current, then a black colour was obtained. Because the colouring was so dark and intense no irregularities could be seen.

EXAMPLE 2 About pre-anodised samples, giving a total surface area of 1 m were successively coloured in an electrolyte, whereby the alloy from which the sample was made, the composition of the electrolyte, the geometrical arrangement and the voltage of the alternating current were as in Example 1.

In the course of colouring, the pH value fell from 4.7 to 4.2; all the samples showed the colour irregularity mentioned in Example 1.

By adding 4 g/l triethanolamine to the electrolyte the pH was raised again from 4.2 to 4.7. Next, another 100 samples were coloured using alternating current and 16 V. After only 90 seconds instead of seconds treatment the same medium bronze tone as in Example 1 was obtained, and all the samples showed a uniform colour on both surfaces. If during the colouring the pH fell to 4.2 it could be increased again by another addition of tri-ethanolamine.

In order to get a black colour of the same intensity as in Example 1 a treatment time of only 10 minutes instead of 15 minutes was necessary.

By adding tri-ethanolamine not only can the original pH value of the colouring electrolyte be restored but a reduction in the colouring time and an improvement in the colour uniformity can be achieved.

This example shows too that the coloured sample first shows the desired uniformity of colour when an addition of tri-ethanolamine has been made. For economic operation of the process at least 1 g/l of an amino-alcohol must be added before beginning to colour articles.

EXAMPLE 3 Pre-anodised samples of the same alloy composition as in example 1 were treated using alternating current in an aqueous colouring electrolyte containing 20 g/l SnSO, and 7 g/l H SO with a pH of 1.4 at 20 25C. When the pH fell to 1.3, 3 g/l of mono-ethanolamine was added. This way not only was the pH value raised to 1.4 but the colour uniformity was improved and the colouring time was shortened. Although, apart from the addition of the mono-ethanolamine, the same conditions as in the normal processes are employed, a particular colour tone from the available range, can be produced faster and in a better quality.

What we claim is:

l. A process for colouring previously anodicallyoxidised articles of aluminum or aluminum alloys by the electrolytic treatment of the aluminum oxide layer with alternating current in an acidic, aqueous electrolyte containing metallic salts for colouring, in which the electrolyte has an addition of an amino-alcohol made to it.

2. A process according to claim 1 in which at least 1 g/l of an amino alcohol is added to a new electrolyte before it is put into service.

3. A process according to claim 2 in which at least 2 g/l of an amino-alcohol is added.

4. A process according to claim 2 in which sulphuric acid is added to correct the pH of an electrolyte, which has been increased too much by a previous addition of an amino-alcohol.

5. A process according to claim 1 in which the pH value which falls during the colouring process, is corrected by an addition of an amino-alcohol.

6. A process according to claim 1 in which the amino-alcohol addition consists of mono-, dior triethanolamine or mixtures of these.

7. A process according to claim 1 in which the electrolyte contains the ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Fe. 

1. A PROCESS FOR COLOURING PREVIOUSLY ANODICALLY-OXIDISED ARTICLES OF ALUMINUM OR ALUMINUM ALLOYS BY THE ELECTROLYTIC TREATMENT OF THE ALUMINUM OXIDE LAYER WITH ALTERNATING CURRENT IN AN ACIDIC, AQUEOUS ELECTROLYTE CONTAINING METALLIC SALTS FOR COLOURING, IN WHICH THE ELECTROLYTE HAS AN ADDITION OF AN AMINO-ALCOHOL MADE TO IT.
 2. A process according to claim 1 in which at least 1 g/l of an amino alcohol is added to a new electrolyte before it is put into service.
 3. A process according to claim 2 in which at least 2 g/l of an amino-alcohol is added.
 4. A process according to claim 2 in which sulphuric acid is added to correct the pH of an electrolyte, which has been increased too much by a previous addition of an amino-alcohol.
 5. A process according to claim 1 in which the pH value which falls during the colouring process, is corrected by an addition of an amino-alcohol.
 6. A process according to claim 1 in which the amino-alcohol addition consists of mono-, di- or tri-ethanolamine or mixtures of these.
 7. A process according to claim 1 in which the electrolyte contains the ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Fe. 